This invention relates to a process for preparing 1-oxa-.beta.-lactam antibiotic compounds. The 1-oxa-.beta.-lactam compounds, also referred to as 1-oxa (dethia) cephalosporin compounds, have been described by Narisada et al., U.S. Pat. No. 4,138,486, Christensen et al., U.S. Pat. No. 4,226,866, and Wolfe, U.S. Pat. No. 4,013,653. The 1-oxa-.beta.-lactam compounds represent a new class of antibiotics possessing interesting antibacterial properties. Current methods employed for the synthesis of these compounds involve multi-steps which are often low-yielding and difficult to control with respect to the desired stereochemistry. Because of these difficulties in the synthesis of the 1-oxa-.beta.-lactam compounds, new and simpler synthetic processes are sought.